Interhemispheric transfer (IT) time through the corpus callosum (CC) can be measured with a manual reaction time (RT) to lateralized visual stimuli (the so called Poffenberger Paradigm) by subtracting mean RT of faster uncrossed hemifield-hand combinations (not requiring IT) from slower crossed combinations (requiring an IT). However, the nature of the signal transmitted by CC is still uncertain. In the present study we wanted to verify whether IT occurs at controlled (i.e. perceptual or pre-motor) or ballistic (i.e. motoric) stages of RT. In a first experiment we employed a stop signal paradigm, that is, a visual RT task in which go-trials are interspersed among stop-trials in which subjects are supposed to refrain from responding following an acoustic tone. We found that crossed stimuli were easier to inhibit than uncrossed stimuli. This suggests that IT occurs at the controlled stage of RT and therefore prior to the point of no return (PNR), i.e. the point beyond which the response cannot be inhibited. Since the locus of the PNR is still uncertain, in a second experiment we used response force as a dependent variable reflecting the activation of the motor cortex. We found that none of the force parameters studied (peak force, impulse size and response duration) differed between crossed and uncrossed stimuli while temporal parameters confirmed the presence of an advantage of the uncrossed combinations.

All together these results suggest that callosal transmission occurs at the stage of controlled processes and rule out the possibility of a IT at the motoric stage.